1. Field of the Invention
The present invention relates to hoist controllers, and more particularly to parallel control systems for coordinating the movement of a plurality of hoists.
2. Description of the Prior Art
Automatic control over hoist movement has had extensive development in the past, particularly in the context of the transverse and extension movements of a single hoist.
Typically hoists suspended on overhead cranes are arranged for lateral translation both along and transverse to an overhead support track with the vertical dimension determined by the extension of the hoist. Since the foregoing axes of motion are substantially orthogonal very little cross-coupling results between the various motions, particularly since the motions are maintained at rates sufficiently low to minimize any cross product terms. Thus the prior art hoist controllers typically take the form of position controllers along the three orthogonal axes which more or less operate independently.
Occasionally, however, overhead cranes are useful to effect suspension alignment in three dimensions. For example, overhead stage lighting often entails the movement of lighting arrays which are sometimes tilted around one or two pivotal axes in the course of such alignment. Thus, occasions arise in which hoists, of necessity, are moved in groups which, in the course of their motion and extension, then define the vertical and angular alignment of the articles supported. To effect such alignment, three or more hoists are typically used in a single grouping and define by the extension therebetween the effective alignment of the article. Thus the extension of the hoists and their consequent angular alignment with respect to the overhead crane determines the effective alignment of the articles suspended. When the mass distribution of the suspended article is not known a closed loop arrangement combining all three hoists becomes mathematically an insurmountable problem. Accordingly, the preference is for effecting hoist control by way of point-by-point observation and it is a technique for queing point-by-point sequences into coherent movements that is disclosed herein.